Best 1 series DSLR that I have owned. Better menus, easier and more logical controls. Great image quality and color fidelity compared to my 1DM2. I will spend much less time post processing with 1DM3 files. Better AF than my 1DM2. AIServo much better than 1DM2

Cons:

Would like to be able to easily save at least 3 sets of registered settings in camera.

I love this camera! I had an original 1D, sold it when I got the 1DM2. Had a 10D, then a 20D as smaller, lighter backup. When shooting both the 1DM2 and the 20D, I got better color with the 20, and had to work hard fussing with white balance of the 1DM2 raws to get the images to look like they came from the same shoot. Also, the 1DM2 gave me a fair number of OOF images even in 1-shot.

I've had no AF problems with the 1DM3. AIServo is fast and accurate. ISO 6400 is amazingly good. I am considering buying another 1DM3 and selling the 20D.